Syrian rebels said on Saturday that the Syrian army and their allies were intensifying attacks on a demilitarised zone in the northwest in an attempt to undermine a Russian-Turkish deal that has averted a major offensive on their last stronghold.

They said the army has stepped up its onslaught with hundreds of mortar and rocket attacks on a string of rebel-held villages and towns in northern Hama, southern Idlib and Latakia that fall within a demilitarized zone agreed last September between Russia and Turkey, Reuters reported.

"The regime has targeted all the fronts in the demilitarised zone. We have responded by striking at their military posts that have struck populated villages and towns," said Captain Naji Abu Huthaifa, a spokesman for the National Liberation Front, an alliance of Turkish-backed rebels.

On Friday, militants killed 22 government fighters near the planned buffer zone, the Britain-based Syrian Observatory for Human Rights was cited by AFP as saying.

One civilian was injured, after a renewed shelling by the regime forces targeted Babulin and Dar AlSalam and Hayish villages, eastern #Idlib. Our #WhiteHelmets teams are working to inspected and secured the target area. pic.twitter.com/nQYNuc2YS4

Russia and Turkey reached a deal in Sochi last September to enforce a demilitarised zone in Idlib and adjacent areas that is the last stronghold of rebels who rose against President Bashar al-Assad in 2011.

Idlib Province is also home to about three million people, more than half of whom have already been displaced at least once during the war.

The Syrian army and allied militias had wanted to press on to regain the last of the rebel-held areas after recapturing southern Syria and ending rebel control around the capital.

Syrian state media, citing army sources, blamed rebels for the attacks and accused them of trying to wreck the Russian-Turkish initiative.

Under the deal, Turkey had pledged to drive out al-Qaeda inspired fighters from the zone, but the Russian military is increasingly questioning Ankara's ability to implement it.

The main militant group, Tahrir al-Sham (HTS), has so far not withdrawn heavy weapons, a regional intelligence source said on Saturday.

Enes Ayasli, a research assistant and Middle East expert at Sakarya University in Turkey, told the Arab News the most obvious setback of the Idlib deal is that moderate rebel groups in the province will now back HTS if there is a clash between it and Syrian forces.

"Their focus is now on repelling regime forces even if it means violating the deal," he told Arab News. "Turkey in this sense seems to have failed to separate moderate groups completely from extremists."

An intensification of fighting between the Syrian government forces and militants may cause the deal to collapse completely, Ayasli said.

Still, despite the flare-up in violence, the Russian and Syrian air forces have not resumed bombing since the deal was agreed.

The pro-government AMN website said the Syrian army is preparing for a large-scale operation in the southeastern countryside of Idlib Province, according to a military report.

The main objective of the operation would be to clear the remaining towns under the control of HTS near a Syrian military installation, AMN said.

The government attacks have forced hundreds of families that had earlier been encouraged to return to some of the frontline villages in the zone to flee farther north near the Turkish border, residents said.